Fix Gitlab::Patch::Prependable to correctly prepend class methods

Problem to solve

Currently, the Gitlab::Patch::Prependable does not correctly prepend class methods although it looks like it was designed to do so.

This leads to explicit prepending of class methods "by hand", for instance in EE::Feature and there are a couple more examples.

Reason for unexpected behavior

Currently unknown

Proposal

A fix is needed